Optical measurements on crude oils for composition analysis had been previously demonstrated by Buckley et al., “Asphaltene Precipitation and Solvent Properties of Crude Oils,” Petroleum Science and Technology, 1998, Vol. 16(3-4), pgs. 251-285. Until recently, very little had been explored with regard to the use of Surface Plasmon Resonance (SPR) measurements for testing reservoir fluids (oil and natural gas). Ooms et al., “Surface Plasmon Resonance for Crude Oil Characterization,” Energy & Fuels, 2015, Vol. 29(5), pgs. 3019-3023 describes Surface Plasmon Resonance in connection with oils.